The present invention concerns the transcutaneous determination of perfusion efficiency in tissue through which blood is being perfused. Such a determination can be performed using a concentration-sensing probe which is laid against the skin surface and serves to ascertain the concentration of a substance, such as oxygen, transported by the perfused blood, along with a perfusionsensing probe which likewise is laid against the skin surface and serves to ascertain the perfusion rate. U.S. Pat. No. 3,918,434 discloses a single probe capable of performing both these measurements, in order to be able to ascertain perfusion efficiency; the disclosure of that patent is incorporated herein by reference, for background information concerning perfusion efficiency and its measurement.
With apparatuses capable of measuring perfusion efficiency, it is frequently necessary to be able to ascertain the maximum limit case of perfusion efficiency, i.e., the case wherein the arterial and transcutaneous concentrations of the indicator substance of interest are equal.
For example, if the delivery of the indicator substance to tissue by the capillaries feeding such tissue is so low that no perfusion-independent concentration field of the indicator substance is formed, then likewise transcutaneous measurement of the arterial concentration of the indicator ceases to be possible, because the arterial indicator concentration is not fully communicated to the skin surface.
Such a critical case can be ascertained by observing changes in perfusion accompanied by simultaneous and parallel changes in the transcutaneously measured indicator concentration.
In the minimum limit case, in contrast, the perfusion disappears, whereas in the maximum limit case the indicator concentration is independent of perfusion.
A perfusion efficiency lower than that of the maximum limit case is also of pathophysiological significance.
Apparatuses which generate the measurement data needed to determine perfusion efficiency--i.e., transcutaneously measured indicator concentration and perfusion--are frequently employed in intensive-care installations. Attempts to use such apparatus in intensive-care installations to monitor perfusion efficiency over long periods of time are very inconvenient and impractical.
Even if the apparatus generating the measurement data needed to determine perfusion efficiency is also set-up to determine perfusion efficiency per se--i.e., as in U.S. Pat. No. 3,918,434--, the problem remains that, in long-term intensive-care contexts, direct observation of perfusion efficiency is not sufficient for the needs of the situation. In particular it is parallel changes in the two parameters (indicator concentration and perfusion) which together characterize perfusion efficiency that are important, and which justify the generation of alarm signals signalling a critical situation requiring emergency counter-measures.